The present invention relates to a color image forming apparatus equipped with a plurality of exposure-scanning units, each of which performs an exposure-scanning operation onto a photoreceptor by means of a laser beam reflected by a rotating mirror, and specifically relates to a method of controlling the abovementioned exposure-scanning operation.
For instance, in a color image forming apparatus employing an electro-photographic method, a full color image is formed by sequentially overlapping a plurality of unicolor images. For this purpose, a method of forming a full color image by transferring each unicolor image onto a transfer sheet every time after forming a toner image to overlap unicolor images on the transfer sheet, and a method of forming a full color image by repeating a uniform charging operation, an exposure operation and a developing operation on a photoreceptor to transfer a full color image onto the transfer sheet at a time after overlapping a plurality of unicolor toner image on the photoreceptor, are available
As for both methods mentioned above, image forming apparatus, equipped with a plurality of exposure-scanning units for performing the exposure operation, have been proposed to shorten the time required for forming a full color image. In one of such image forming apparatus, a plurality of units, such as chargers, the exposure-scanning systems, and the developing devices, are disposed in the periphery of the photoreceptor to form a full color image on the photoreceptor and to transfer it on the transfer sheet. In another one of such image forming apparatus, a plurality of photoreceptors are disposed in the periphery of a conveyance device for conveying the transfer sheet, and unicolor images, which are formed by means of a plurality of units, such as chargers, the exposure-scanning systems, and the developing devices, disposed opposite each of photoreceptors, are sequentially transferred onto the transfer sheet.
Incidentally, to form a full color image, it is necessary to perform exposure operations for three primary colors or four primary colors. For this purpose, it is also necessary to precisely adjust the exposure timing so that each unicolor image overlaps on the other unicolor images at the accurately same position, in such image forming apparatus.
The methods for controlling the exposure timing, in a manner as the above, are set forth in, for instance, Tokkaisho 62-266575 and Tokkaisho 62-295084. According to the method for the laser printer disclosed in Tokkaisho 62-266575, the exposure-scanning operation by the laser beam for every color starts when a counted value of clock signals reaches the predetermined set-value, and when the scanning start point deviates from the reference position, a deviation amount is detected and the predetermined set-value is changed, corresponding to the deviation amount. On the other hand, according to the method for the color copier disclosed in Tokkaisho 62-295084, the microprocessor controls the commencement timing of the exposure operation by counting a number of pulses having a predetermined period, while the interruption having a relatively long interval is activated until the time just before the generation of the timing signal, to relieve the workload of the microprocessor, and the interruption having a short interval of the timer set-value is activated once just before the generation of the timing signal, to precisely generate the timing signal.
In conventional color image forming apparatus, deviation amounts are detected in a manner as mentioned above, and the scanning start positions are changed, corresponding to the detected deviation amounts, to coincide positions of the unicolor images to each other. However, since the exposure-scanning operation for each of the unicolor images is performed under the independent control system, it has been difficult to eliminate positional deviations, even if the above-mentioned changing operation for the scanning start positions is performed, and therefore, it has been difficult to produce a high-quality color image due to occurrences of the color deviations.
To overcome the abovementioned drawbacks in conventional color image forming apparatus, it is an object of the present invention to provide a color image forming apparatus, which makes it possible to form a high-quality color image without generating any color deviations.
Accordingly, to overcome the cited shortcomings, the above-mentioned object of the present invention can be attained by the color image forming apparatus described as follows.
(1) A color image forming apparatus, comprising: a photoreceptor; a plurality of exposure-scanning sections, each of which performs an exposure-scanning operation onto the photoreceptor by means of a laser beam deflected by a polygon mirror and includes a horizontal-sync. signal generator to generate a horizontal-sync. signal synchronized with a rotational period of the polygon mirror, wherein one of the exposure-scanning sections outputs a reference horizontal-sync. signal; a synchronizing section to synchronize the horizontal-sync. signal, outputted by the other exposure-scanning section, with the reference horizontal-sync. signal, and to synchronize a image tip signal with the reference horizontal-sync. signal; an adjusting section to adjust a commencement of the exposure-scanning operation performed by each of the exposure-scanning sections, based on the reference horizontal-sync. signal outputted by one of the exposure-scanning sections; and a positional deviation detecting section to detect a positional deviation from a plurality of marks, formed on the photoreceptor by means of the exposure-scanning sections, in a state of synchronizing the horizontal-sync. signals each other and synchronizing the image tip signal with the reference horizontal-sync. signal.
(2) The color image forming apparatus of item 1, wherein the adjusting section adjusts a positional deviation in a unit of one scanning line by performing an adjustment between the horizontal-sync. signal and the image tip signal, and adjusts a positional deviation less than one scanning line by changing a phase of clock signals utilized for driving the polygon mirror, based on data of the positional deviation detected by the positional deviation detecting section.
(3) The color image forming apparatus of item 2, wherein a unit adjustment for changing the phase of the clock signals, utilized for driving the polygon mirror, is greater than an amount of jitters generated by the polygon mirror.
(4) The color image forming apparatus of item 3, wherein an adjusting amount for changing the phase of the clock signals, utilized for driving the polygon mirror, is in a range between zero and [(one scanning line/the unit adjustment)xe2x88x921]xc3x97the unit adjustment.
(5) The color image forming apparatus of item 1, wherein each of the marks includes at least a line segment being in parallel to a main scanning direction of the laser beam.
(6) The color image forming apparatus of item 1, wherein the exposure-scanning sections are disposed opposite the photoreceptor.
(7) The color image forming apparatus of item 1, further comprising: a plurality of photoreceptors, serving as the photoreceptor, wherein each of the exposure-scanning sections is disposed opposite each of the photoreceptors.